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ASEMaster35yrs, Chevy Technician
Category: Chevy
Satisfied Customers: 2090
Experience:  ASE Certified since 1972,Master Auto tech,Heavy Truck Tech ,chev dealer tech
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tell me the symptoms of a bad cam position sensor on the 2001 496 HO?

Resolved Question:

Can you tell me the symptoms of a bad cam position sensor on the 2001 496 HO?
Submitted: 3 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 3 years ago.
a worn cam can cause an issue with the lifter rattle,engine miss fire and/or back firing through the intake manifold
Customer: replied 3 years ago.
I was referring to a bad/faulty cam position sensor.
Expert:  Tim's Auto Repair replied 3 years ago.
sorry i was offline
the common thing that a bad cam sensor will do,,,is cause long starts...the starter will turn the engine allot to before it will run....most times,,a bad cam sensor will give a trouble code
2001 Chevy Truck C 2500 Truck 2WD V8-8.1L VIN G
Camshaft Position Sensor Description and Operation, Components

Camshaft Position Sensor

The Camshaft Position (CKP) sensor works in conjunction with 1x reluctor wheel mounted at the rear of the camshaft. The CMP is used to determine the top dead center position of cylinder #1, and will synchronize with the 24x CKP sensor signal for quicker starting. The PCM will detect an increase in the Mass Airflow (MAF) sensor signal when the engine starts. The CMP signals are output as a digital waveform.

Customer: replied 3 years ago.
I need more info than that. I read what it said in the alldata notes. Right now it doesn't crank at all. Sometimes it will crank, on the first turn of the key. It appears to be really rich. It sometimes shuts down.
Expert:  Tim's Auto Repair replied 3 years ago.
tell me....does it seem to be rich when the engine is hot and cold?
does the engine light stay on?if so,can you give me the computer codes?
explain to me when and how it shuts down...
tell me all you can
Customer: replied 3 years ago.
It is a marine application. It was running fine for 30 minutes, then just shut down instantly. Wouldn't crank back. Cranked the next day but after a few minutes, it would shut down again. Progressively getting worse. Replaced the crank sensor, no improvement. It will hit instantly sometimes for a second but not much. It will backfire sometimes when trying to crank. The crank sensor will not go all the way in to the flange but if it goes in that far, the bolt will not line up. All the wiring has been checked. Mostly when trying to crank, the tach does not move after the initial bump. The once in a while, it will crank. The back of the boat is way blacker than in the past. It seems that even when it won't crank, fuel delivery is fine because it really smells up the shop.
Expert:  Tim's Auto Repair replied 3 years ago.
this does sound as if it does have a bad crank is very possible to get a bad new one

lets also check the fuel pressure,,these must have no less than 55psi at all times..the pump may be sticking and weak

also, the ect[engine coolant tempeture sensor could cause it to run well when cold,,then,act up this way when it gets warm......
i dont know your aplication,,but,if you can scan the computer to getthe trouble codes,,these codes may give a clue to your problem........
note"most times a bad crank sensor wont give a code if they do this

to answer the question,,,it does not sound as if it is a cam sensor problem

Customer: replied 3 years ago.
the crank sensor is pulsing the ecm, even so, there is no tach movement.
Expert:  Tim's Auto Repair replied 3 years ago.
i will opt out here...
one of the other guys will be right with you that knows more than me about this boat 8.1 aplication,,,,,,
someone will be right with you
Customer: replied 3 years ago.
Ok, I had no luck with the boat guys but I will wait to see this time.
Expert:  Michelle-Mod replied 3 years ago.

Hi, I’m a moderator for this topic. Your Professional has opted out and I wonder whether you’re still waiting for an answer. If you are, please let me know and I will do my best to find another Professional to assist you right away. If not, feel free to let me know and I will cancel this question for you. Thank you!

Customer: replied 3 years ago.
still waiting
Expert:  ASEMaster35yrs replied 3 years ago.

Hi, glad to be of help. This sounds to me it is a bad connection to/from or bad ignition module or coil pack. A bad cam sensor will affect injector pulse. You may also have a bad reluctor wheel for the crank sensor but not probable. Being that you are not getting a tach signal, the problem lies in this area. Check injector pulse with a noid light and check at coil negative for a flash pulse while cranking. If you are getting injector pulse the problem is not cam sensor. If you are not getting or intermittent flash pulse at coil negative while cranking, the problem is ignition module or crank sensor. If you are getting steady flash pulse, the problem is coil pack. If you are not getting injector pulse, the problem is cam sensor or ECM. If you are getting flash pulse at coil negative and no injector pulse, the problem is ECM.


Symptoms of a bad camshaft

Typically popping through the carb is a tell tale sign. Loud rockers (tapping is a sign).

popping through the exhaust is also a sign. If a lobe went flat then you would have lifter, push rod, and rocker clacking pretty good. 1st symptom is you have a faint tap and need to adjust the tappet. Put a vacuum gauge on the motor some where and see if there is fluctuation in the needle if there is very possible cam problem. You can pull the valve covers and crank it and see which, if any, rockers don't move or only move slightly. Backfiring through the carb, increasing rocker noise(single), and from memory, all this goes from noticeable to real bad in about 50 miles.

Customer: replied 3 years ago.
I don't really know where the ignition module is. I though it had 8 seperate coil packs all powered from the ecm. We didn't have a light for the injectors but assumed they were fine, because when we try to crank it, the shop fills up with gas fumes. We did replace the crank sensor but it has not cranked but once with the new sensor in. I just assume we need to check the signal wire from the crank sensor to the ecm. I'm not sure if it signals back with a +5v or ground signal though. And then check the coil connections?
Expert:  ASEMaster35yrs replied 3 years ago.

There are two different coil pack set ups and not sure which one you have, but ignition module is directly underneath the coil pack. The magnavox type coil pack is one unit, the other type is individual, usually one coil per two cylinders. If you have indiviual coils for each and every cylinder, you are correct, the ECM performs all the fucntions and there is no ignition module. Crank sensor relates to spark at the plugs and cam sensor relates to fuel injectors firing.

Get a noid light to check injector pulse. It looks like this:


They make different ones for different applications. You can get a noid light set here or at any auto parts store or Sears.,or.r_gc.r_pw.r_q f.,cf.osb&biw=1920&bih=907&ion=1&wrapid=tljp133944524392708&um=1&ie=UTF-8&tbm=shop&cid=3766586809689387754&sa=X&ei=_U_WT-PSFsPr0gGmo7j_Ag&sqi=2&ved=0CJoBEPMCMAU


I just checked on your system and you have individual coils, one on each cylinder and you are correct, the PCM does it all. You need to check your circuits. Here are the diagrams:

Cam & crank sensor circuit diagram

Ignition coils


If your circuits check out, replace the ECM.

Here's some videos for testing your cam & crank sensor circuits:

Crank sensor circuit testing

Cam sensor circuit testing

Cam & crank sensor testing

The symptom of a bad cam sensor is no injector pulse.

ASEMaster35yrs, Chevy Technician
Category: Chevy
Satisfied Customers: 2090
Experience: ASE Certified since 1972,Master Auto tech,Heavy Truck Tech ,chev dealer tech
ASEMaster35yrs and other Chevy Specialists are ready to help you
Expert:  ASEMaster35yrs replied 3 years ago.
Thank you very much for your most positive feedback and accept. Thank you for using just answer.
Customer: replied 3 years ago.
Evidently during some startup sequence, the pcm will fire #2 and #7 and then wait for the cam switch input. Because I checked the cam wire and it stayed hot 5v the whole time. I removed the cam switch and wiped some junk from the end, fired right up. I am still going to replace it with a new one rather than chance it. Thanks for your help.
Expert:  ASEMaster35yrs replied 3 years ago.
You're very welcome, glad you found the problem. If you have more questions you can contact me at

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